1. Field of the Invention
The present invention relates to a semiconductor memory device and a method for fabricating the same, and more particularly, to a cell structure of an erasable programmable read only memory (EPROM) device and a method for fabricating the same.
2. Description of the Related Art
Semiconductor memory devices can be classified into different types. Among them, there are random access memories (RAMs) and read only memories (ROMs). When the power supply to a memory is interrupted, stored data in the RAM is erased while the ROM maintains stored data. For this reason, memory devices such as ROMs are called “nonvolatile memory devices.” EPROM devices, which belong to these nonvolatile memory devices, are reusable by erasing stored data.
Stored data in the EPROM device can be erased by applying ultraviolet rays to a window attached to the surface of a memory chip. Research is being conducted to develop system-on-chip (SOC) devices, in which the EPROM device has been used along with various devices, such as logic circuits or drive circuits, to manufacture these SOC devices. The EPROM device, which can effectively alter fabricated chips, is employed to calibrate minute disparity between products or represent manufacturers' names on the fabricated chips.
In processes of fabricating semiconductors, as the line width of circuits becomes finer, silicide layers, such as TiSix, CoSix, and NiSix, are used to reduce resistances of gate lines and metal contacts. When a silicide layer is used, a nitride layer is used instead of an oxide layer as a gate spacer in order to improve characteristics of a transistor by preventing a short between a gate and a source/drain during a silicidation process.
When a cell of the EPROM device is formed by using the nitride layer as the gate spacer, an ONO (oxide-nitride-oxide) layer is formed on top of a floating gate, and a nitride spacer is formed on the sides thereof. That is, nitride layers are formed all around the floating gate. In this case, since ultraviolet rays cannot penetrate through the nitride layers, it becomes difficult to erase charges of the floating gate by ultraviolet rays. Thus, unlike the case where an oxide layer is used as the floating gate spacer, the EPROM device cannot perform erase operations using ultraviolet rays.